I. Field of the Invention
The present invention relates to an improved switching transistor inverter circuit for driving high power AC loads from a DC source and more particularly, to a switching amplifier circuit for use with such a switching transistor inverter circuit for operating a linear metal halide type of arc discharge lamp.
II. Description of the Prior Art
A bridge inverter has several advantageous features which make it a desirable circuit for operating arc discharge lamps. The first of these features is the peak working voltage requirement on each leg of the bridge. In the bridge inverter, the peak leg stress is equal to the DC source voltage level. This can be contrasted with push-pull inverters and series parallel types which impose a 2X or more multiple of the DC source as the peak working stress on the inverting switches. When the DC source approaches the maximum withstanding voltage capability of available transistor devices, the bridge inverter circuit must be considered for use. Another desirable feature is the way in which loads can be driven with non-symmetrical forward and reverse conduction periods to produce an AC+DC conduction. Transformer coupled load inverters do not have this inherent advantage, although they do constitute the mainstream of products in DC to AC power conversion.
If the power transistors in each leg of the bridge inverter are operated as switches, that is, fully saturated when ON at the maximum load current level and biased into cut off when OFF, then large amounts of power can be controlled in the load with relatively little power loss in the inverter. Furthermore, if one were to keep the switching transition time very short compared to the period of switching, very little power would be lost in the transition and high speed reversal of conduction in the load would be the result. This is essential to keep an arc discharge lamp of the linear metal halide type in the ON state since the lamp has a very short deionization time.
In the use of a switching transistor bridge inverter circuit, there arises the problem of synchronization of the switching of all four legs thereof. Each diagonal crosspair as for example Q1 (FIG. 1) and Q3 must work as one switch, switching on and off together. For reliable operation especially at high voltage and current levels, it is important that switchover of conduction from one crosspair to the opposite crosspair occur rapidly. Furthermore, it is imperative that during switchover a vertical leg pair not be biased on with momentary full conduction in the vertical pair as Q1 and Q2. This is a condition described in the art by many terms such as overlap, switchthrough or shoot-through. The consequence is rapid degradation of the transistor pair, if not immediate catastrophic failure. Circuit designers must avoid this condition by designing and/or adjusting in a determinate amount of load current flank time, dead time or zero time. Another approach is to use circuitry to sense the state of the power switches during the switchover interval and with combinatorial logic modifying the base drive signals to the power transistor legs so that simultaneous vertical pair conduction can not occur.
It is desirable when operating a load such as a linear metal halide arc discharge lamp from a bridge inverter circuit to provide simple, electrically isolated, DC coupling of drive logic signals to each of the power legs of the bridge to permit AC with DC component conduction in the load and wherein load conduction of one polarity may be for an indefinite period of time. It is also desirable to provide a safe minimum of load current zero time which factors in variations due to circuit manufacturing tolerances and component variations due to temperature, voltage, current and aging. It is also desirable to eliminate all adjustment means such as potentiometers and the like that have been used to compensate for switching time tolerances among the legs of the inverter bridge.
It is an object of the present invention to accomplish all the above with a minimum of parts and in an efficient and reliable fashion.